Slave units in distributed industrial control systems are connected to master control units in chains such that they can be controlled centrally. Each slave unit has a first communication port via which commands are received, processed and exchanged to a second communication port to communicate with further slave units in the chain. Communication ports in a slave unit are bidirectional, and are configured such that an input of the second communication port is communicatively connected to an output in the first communication port. This enables information generated down the chain to be sent in reverse direction up the chain. Also commands which have been sent down the chain in forward direction can be sent back for forwarding data output by the slave units to the master control unit via the reverse direction. To that end the last slave unit in the chain, which has no further connection to another slave unit, has a closed second communication port which loops back the commands to the first communication port by which the command entered the slave unit.
A problem with forwarding commands in a distributed control system having a chain of slave units according to the state of the art is that when an interruption in the communication occurs in the slave unit chain, such that the chain breaks up in to two sections, only the first section before the interruption receives the commands, whereas the second section after the disconnection or interruption no longer receives commands. This causes potential failure of the control system as a whole, as cooperation of parts of the chain of slave units may be essential.
In the state of the art, it has been established that the last slave unit of a chain of slave units can be connected to a second port of the master control unit, instead of closing the last communication port of the last slave unit. Commands need no longer be looped back through the chain in reverse direction, but are received at the second communication port of the master control unit. Thus a redundant path within the chain of slave units is created.
When an interruption in the chain of slave units occurs, the slave units at both sides of the interruption close their bordering ports such that two sub chains are formed, wherein one sub chain receives its commands via the former route by forwarding commands down the sub chain and back in reverse direction, whereas the second sub chain receives commands form the second communication port of the master control unit via the reverse path first and looped back via the forward path to the second communication port, thereby utilizing the redundant path in reverse direction.
A disadvantage of this approach is that the master control unit needs to utilize two communication ports, to create the redundant path in a single chain of slave units, i.e. one communication port for each sub chain after an interruption in the communication, where communication ports may be scarce.